The present invention relates to an apparatus for measuring the flow rate of fluid media with a rod-like interfering or dislocating body rigidly arranged in the flow having a cylindrical flow-against surface on which periodically separating eddies form and a device with a sensor for recording the eddy separating frequency which is dependent on the flow rate.
Apparatuses of the present construction are based on the principle of the Karman votex or eddy path. The phenomenon was discovered by Karman on cylindrical rods and was investigated from the flow standpoint. In the case of turbulent flow on the blind side of the cylindrical rod eddies form on opposte sides, breaking away or separating alternately on the two sides and migrating downstream. The eddy separating frequency can be measured with sensors of different types, e.g. pressure transducers, hot-wire sensors, etc. The eddy separating or breaking away frequency is a measure of the flow rate of the fluid medium, siad effects occurring in both gas and liquid flows. In a certain range of the Reynolds number the separating frequency is proportional to the flow, so that in the case of corresponding dimensioning and arrangement of the interfering body in the flow, a troublefree and precise measurement is possible.
Outside the aforementioned Reynolds number range dynamic distrubances from the flow are superimposed on eddy or vortex formation, so that in said ranges there is no longer any proportionality between the flow and the separating frequency. In order to be able to still utilize this effect for flow measurement purposes in these ranges, particularly in the case of high Reynolds numbers, in the past attempts have been made to optimize the shape of the interfering body in such measuring equipment so as to obtain an at least reproducible dependence of the separating frequency on the flow rate over a larger Reynolds number range. Thus, it is known to provide the interfering body in the form of a half circular cylinder with a cylindrical flow-against surface or to use a prismatic interfering body, which extends downwards from an upstream linear edge with planar flow surfaces (DE-OS No. 20 38 569). The increased flow resistance of such an interfering body is to be reduced by following assemblies, such as transversely positioned discs and the like. It is also known (U.S. Pat. No. 3,116,639) to replace the solid cylindrical interfering bodies by those having a partly elliptical flow-against surface and to record the breaking away or separating frequency by an oscillatably mounted sensor positioned in aligned manner behind the interfering body. It is also known (German Patent No. 19 26798) to arrange in the flow a plate-like baffle surface substantially at right angles thereto and which has a symnmetrically fixed, downstream extending stabilizing surface. All the known constructions suffer to a greater or lesser extent from the disadvantages referred to hereinafter.
A considerable pressure loss results from the shaping of the interfering body. In addition, the effective pipe cross-section is considerably limited by the interfering body. Measurement is impaired by pulsations in the flow, which are caused by flow generators, pipe bends, assemblies such as valves and the like, because the flow profile is disturbed and in certain circumstances this can lead to brief breaking away effects in the vortex path. This once again leads to measuring errors. In order to avoid this to a certain extent, it is necessary to provide a long intake section with straightening blades in order to obtain a uniform speed profile in the case of turbulent flow. For many measuring functions it is not possible to realise such intake sections. The shape and dimensioning of the interfering bodies lead to wall influences, which also falsify the measured result. From this is derived the requirement to construct the measuring body size in a particular ratio to the pipe diameter, so that a corresponding number of interfering bodies must be made available for the different nominal pipe bores.